A successful hip replacement or arthroplasty procedure results, in part, from selection of prosthetic joint components that are dimensioned and positioned to closely approximate or replicate the geometry and functional characteristics of a natural, healthy hip joint. Typically, the component selection process includes a pre-operative analysis of joint images. The component selection process also includes temporary fixation of one or more provisional components to a bone or bones of interest prior to permanent fixation of the prosthetic joint. The provisional components are intended to mimic certain aspects of the permanent prosthetic joint in order for a surgeon to validate measurements and to test or "tryout" several different component sizes and configurations. Hence, provisional components are aptly known as "trials."
In a known procedure, a trial for a hip femoral component is used in the following manner. The proximal end of a femur is resectioned and the medullary canal of the femur is reamed. A broach is inserted into the resectioned proximal end of the femur to provide a cavity within the bone dimensioned and contoured to receive a femoral stem. Prior to removing the broach, a trial neck or trunnion and trial head can be secured to the broach to simulate a complete femoral stem. Normally, several neck and head trials of varying lengths and geometries are successively joined to the broach in an attempt to determine an appropriate neck length and overall femoral stem length. Once these lengths have been determined, the trial neck and head are removed from the broach and the broach is removed from the femur. Subsequently, a femoral stem of the appropriate length is selected for insertion into the cavity defined by the broach using techniques known to those skilled in the art.
Two types of femoral prostheses are typically suitable for hip arthroplasty procedures. One type is a bipolar prosthesis. In general, a bipolar hip prosthesis includes a shell having an external surface which articulates with the acetabulum and an internal surface which articulates with the spherical head member of a prosthetic femoral component. The other type of prosthesis is often referred to as a unipolar endoprosthesis in which the prosthetic femoral component includes a spherical head member which is large enough to articulate directly with the acetabulum.
U.S. Pat. No. 5,156,626 describes a procedure used to implant a bipolar hip prosthesis utilizing a four piece trial reduction system. This system includes a femoral broach trial, a neck portion attached to the broach, a head trial attached to the neck, and a shell for receiving the head trial and fitting within the acetabulum of a patient. The trial procedure using these four pieces can be at times cumbersome for the physician because the head can tend to dislocate and move out of the shell when the physician is attempting to place the trial into position. In addition, the trial system requires the use of numerous parts that must be selected and mated in various combinations.
Unipolar trial systems used in implanting unipolar hips typically comprise a broach, neck, and head trial. Unipolar trial counterpart pieces could be used in some situations to perform the trial reduction of bipolar hip implants because one of the primary interests in performing a trial reduction in both bipolar and unipolar implant procedures is to determine device fit (i.e., shell or head) in the acetabulum. Also, in many cases, the range of motion of the bipolar implant can be approximated with a unipolar trial.